1. Field of the Invention
The present invention relates to a process for the controlled etching of tapered vias in borosilicate glass dielectrics.
2. Description of the Prior Art
It is known to use low temperature, low pressure plasma deposited silicon nitride layers as a mask in reactive ion etching technology (RIE) technology; however, the silicon nitride layer is used primarily as a mask and/or passivation layer to prevent oxidation and solder bridging between metallurgical lines over portions of the device reserved for active devices. Typically, some silicon nitride is removed prior to oxidation by reactive ion etching (RIE) which leaves a hole with essentially vertical sidewalls. Sloping sidewalls are avoided because of a "birds beak" problem, and the silicon nitride layer is not silicon rich and nitrogen deficient.
IBM Technical Disclosure Bulletin Vol. 24, No. 11A, April 1982, pages 5547 and 5548 discloses reactive ion etching of silicon nitride.
U.S. Pat. No. 3,648,125 Peltzer discloses the use of silicon nitride for diffusion masks and also for forming recessed isolation regions.
U.S. Pat. No. 4,053,351 DeForest et al discloses a method of chemically machining glass and single crystal, polycrystalline and amorphous forms of silica and like materials using acid fluoride etchants.
U.S. Pat. No. 4,247,361 Shaheen discloses the use of a Teflon film as a masking element in semiconductor manufacture.
U.S. Pat. No. 4,299,862 Donley discloses a process of etching windows in a glass coating where a precisely etchable silicon nitride coating can be initially applied and windows precisely etched therein.
U.S. Pat. No. 4,293,376 Weingand discloses a method for manufacturing a perforated glass plate for use in a gas discharge display by providing an etch-resistant coating on a glass plate and subjecting and thus coated glass plate to gaseous hydrofluoric acid.
IBM Technical Disclosure Bulletins Vol. 19, No. 8, January 1977, pages 3039 and 3040 and Vol. 25, No. 1, June 1982, pages 304 and 305 disclose, respectively, barrier layers for multilayer lift-off masks and electron beam mask writing at high exposure doses.
It has been found that standard photoresist masking technology cannot be utilized to produce via holes in multicomponent borosilicate glass layers of substantial thickness. The reason for this is that commonly used organic photoresists for semiconductor fabrication are not chemically durable enough to withstand and attack of etchants as are typically used for complete dissolution of borosilicate glass layers of substantial thicknesses, e.g., on the order of 10.mu.. Although the patterned photoresist is not soluble in the etchants, it floats away from the borosilicate glass dielectric surface (because the HF is known to attach at the interface) in a short period of time, e.g., on the order of 45 seconds, when exposed to the etchant, allowing the borosilicate glass dielectric layer to be attacked in undesired areas, thereby creating pinholing problems and problems with undesired variations in borosilicate glass thickness, both of which could lead to electrical problems in the final thin film redistribution-metal fan-out lines on the glass-on-ceramic surface.
As the resist is loosened as above, the side walls of the via holes become very tapered, i.e., the angle of the hole becomes very shallow because of excess undercutting of the resist, the preferred angle being about 45.degree., and the taper is uncontrollable.